The following table shows how much water is lost due to gassing in case of a relatively new low antimony battery (gassing increases with age):
Battery (fully charged)
V / cell
Batt. V
Gas generation per 100 Ah battery capacity 20 cc / h 25 cc / h 60 cc / h 90 cc / h 150 cc / h 180 cc / h 500 cc / h
Water consumption per 100 Ah battery capacity
Topping up interval
Water lost per charge cycle
Ah ”lost” per 100 Ah batt. capacity
Open-circuit
2.13 2.17
12.8
0.1 l / year 0.1 l / year 0.3 l / year 0.4 l / year 0.6 l / year 0.8 l / year 2.2 l / year 4.2 l / year 6.5 l / year
5 y 5 y
44 / y 54 / y
Float Float Float Float
13
2.2
13.2 13.5 13.8
1.5 y
130 / y 200 / y 300 / y
2.25
1 y
2.3
10 m
Absorption Absorption Absorption Absorption
2.33
14
7 m 3 m
2 cc 3 cc 4 cc
2 / cycle 3 / cycle 4 / cycle
2.4
14.4 14.7
2.45
1 l / h
2.5
15
1.5 l / h
Gas generation and water consumption is based on a 6 cell (= 12 V) battery. The topping up interval is based upon 0.5 l of water lost per 100 Ah. The water surplus in the battery is approximately 1 l / 100 Ah.
The formulas: a) 1g of water can be decomposed into 1.85 l of oxygen + hydrogen gas b) 1 Ah “lost” due to gassing generates 3.7 l of gas in a 6 cell (= 12 V) battery
The table shows that a float voltage of 13.5 V (13.5 V is an often recommended float level for the flooded batteries under consideration here, as lower float voltages do not completely compensate self- discharge) or higher will result in topping up needed more than once a year. Please also note that batteries with more antimony doping will consume 2 to 5 times more water! To my opinion, instead of trying to find a delicate balance between insufficient voltage to compensate for self-discharge and to much gassing at a higher voltage, it would be better to leave the battery open circuited and recharge, depending on temperature, at least once every 4 months, or to reduce float voltage to a very low level, for example 2.17 V per cell (13 V respectively 26 V), and also recharge regularly at a higher voltage. This regular refreshing charge should be a feature of the battery charger. See section 5.3.2. 2) All VLRA batteries mentioned can be float charged for long periods of time, although some studies have shown that a treatment similar to the one proposed here for flooded batteries will increase service life (see for example “Batterie Technik” by Heinz Wenzl, Expert Verlag, 1999). When not charged sufficiently, batteries will deteriorate due to the following reasons: - sulphation - stratification (flooded batteries only) - cell unbalance, (see sect. 2.5.6). Batteries will in general reach their fully charged state, including equalization, during the absorption charge or when float charged for a sufficiently long period of time. If they have been used in partial state of discharge mode for some time, they will recover by: - repetitive cycling and charging with the appropriate absorption voltage and time - an absorption or float charge during a longer period of time - a real equalization charge, see below. An equalizing charge is done by first charging the battery as usual, and then continue charging with a low current (3 % to 5 % of its Ah capacity, i. e. 3 to 5 A for a 100 Ah battery) and let the voltage increase to 15-16 V (30-32 V for a 24 V battery) until the specific gravity (SG) stops increasing. This will take 3 to 6 hours and by then all cells should give the same reading. Be sure to isolate the battery from all loads sensitive to over-voltage during this period. Especially heavy-duty traction batteries may need a periodic equalization charge.
4.3. Equalizing
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